The introduction of the video display terminal has brought on a substantial change in the office, school and home environment. These new, labor saving devices usually consist of a TV-like cathode ray tube attached to a keyboard and linked to a computer.
Not very many years ago, video display terminals were used on a regular basis only by computer professionals and in limited speciality fields such as air transportation and typesetting. In the recent years the use of such display devices has experienced a rapid growth to the extent that in 1985 it was estimated that there were about thirteen million video display terminals in use in the United States and Canada alone. The phenomenal growth in the use of such devices has brought a parallel growth in the number of people using such devices. It is estimated that 40-50% of American workers will be making daily use of video display terminals by 1990 at more than thirty-eight million video display terminal stations in factories, schools, offices and homes.
A growing number of children are now using video display terminals in the class room as computers become an integral part of every school program. In addition, the children have extensive use of video display terminals for recreational purposes, such as video games and home computers.
The increased use of video display terminals has several side effects that need consideration. Included in those effects are X-ray radiation given off as a secondary emission from the impact of high speed electrons on the viewing screen, ultraviolet radiation and visible light given off from the excitation of the phosphors on the inner surface of the screen, infrared radiation produced from the heat generated on the viewing screen by the impact of electrons and the excitation of the phosphors, pulsating very low frequency fields (VLF) at 15-20 Khz generated by the electrical pulses which cause the beam to go "on" and "off" eleven million times a second, radio frequencies generated by pulsations coming from the coordinating circuits, extremely low frequencies (ELF) of 30-60 Hz produced from the electrical pulses which cause the beam to move vertically in order to refresh the full frame 30 to 60 times a second, and static electricity produced as a result of electrons being ejected from the electron beam.
During recent years, evidence of video display terminal related illnesses has surfaced and an increasing number of health complaints have been reported to the National Institute of Occupational Savety and Health (NIOSH) in the United States. The complaint rate has reached a level where video display terminal operators are number one on OSHA's complaint list. These health complaints have initiated an increase in research into the possible health effects of specifically radiation emissions from the video display terminal. The present stage of this research indicates that the health impact most likely is generated by the high levels of nonionizing radiation emitted from video display terminals. A number of scientific studies have shown that such radiation is biologically very active and has serious adverse impact on animal embroys, as well as creating a significantly higher cancer rate in animals and humans.
It is also observed that the low level radiation and the extremely low level radiation emitted from video display terminals has a very high electric component and a much lower magnetic component.
Today, several scientists recommend shielding the video display terminals for such radiation. Among others, this includes Dr. H. D. Sharma, University of Waterloo, Toronto, Canada and Dr. Art Guy, Professor at University of Washington, Seattle, Washington, in a report to International Business Machine Corporation.
It has been shown that placing a suitable filter on the face of the cathode ray tube of a video display terminal can be effective in reducing or eliminating the radiation and glare problems above described.
While many of the present manufacturers of video display terminals provide the display terminal with a filter screen that can be effective in reducing glare and radiation, there are still a great number of display terminals in use that were never equipped with such filters. Further there are still display terminals being sold that are not equipped with filters. There is therefore a need to provide a system for installing effective filters on unfiltered systems.
Retrofitting a filter to an existing video display terminal is complicated by the existance of secure enclosures enclosing the display terminal and its associated electronic components. Further, some manufacturers have restrictive warranty policies that effectively prohibit a user or owner of the terminal from opening the enclosure to attach a filter to the terminal from within the enclosure. It is therefore desireable to provide a filter and a method for its installation that can be accomplished from the outside of the enclosure for the terminal. The filter must be able to be installed in a manner that will not interfere with the display area of the terminal and will not degradate the display quality while being effective to reduce or eliminate glare and radiation from the terminal.
Prior art filter devices are illustrated in U.S. Pat. Nos. 4,253,737 E. K. Thomsen et al; 4,468,702 L. H. M. Jandrell; and 4,661,856, P. G. Schnack. These patents describe the need for and the formation of filter screens that can be installed behind the bezel of a video display terminal as the device containing the terminal is being manufactured. Those patents do not address the problem of adapting and installing a filter to an existing terminal from the exterior of the housing of the terminal.